Process of producing sodium acid pyrophosphate



Patented Dec. 21, 1943 PROCESS OF PRODUCING SODIUM ACID PYROPHOSPHATEFrank H. Perrln and Otis w. Allen, Pulaski, Tenn.,

assignorsto International Minerals & Chemical Corporation, Chicago,Ill., a corporation of New York No Drawing. Application September 14,1940,

Serial No. 356,834

Claims.

This invention relates to the production of pyrophos'phates that areparticularly suitable, for example, for leavening agents or bakingpowders.

In carrying out the invention, sodium acid pyrophosphate, NazHzPaOr, ora double salt of sodium acid pyrophosphate-monosodium phosphate,Na2H2P2O72NaH2PO4, or a physical mixture of the two salts. is produced.It is immaterial in carrying out the process whether the result is adouble salt or a physical mixture of the two salts.

Heretofore, acid sodium pyrophosphate has been produced by heatingmono-sodiumphosphate, NaH2PO4, at about 200 C. for about six or eighthours in accordance with the formula 2NaH2PO4=Na2H2PzO7+H2Q ,Manymechanical difficulties have been encountered in attempts to producemono-sodiumphosphate, and corrosive-proof containers such as stainlesssteel are needed. It is difficult to dry the product to prevent it frommelting or fusing. The mixture becomes very sticky at elevatedtemperatures and is inconvenient to stir or handle. Also, when stored itcakes into a very hard solid lump in a few days.

By the present invention prior difficulties are overcome and sodium acidpyrophosphate is produced in a rapid and satisfactory way atcomparatively low cost without encountering many of the mechanicaldifficulties heretofore experienced.

The following examples are given to illustrate the invention, but it isto be understood that the invention is not limited to the details given.

Example I Tetrasodium pyrophosphate of any fineness, finely ground beingpreferred, is reacted with phosphoric acid, at room temperature, withthe heat of reaction being all that is required to complete thereaction, using one mol of the former to two mols of the latter:

This reaction is reversible only when the compounds are in solution. Thephysical condition of the double salt (or mixed salts) will be dependenton the strength of phosphoric acid used, but in all cases some drying,at less than the temperature required to convert monosodium phosphateinto sodium acid pyrophosphate, is required so that the final productwill be in such physical condition as to permit grinding into a powder.The preferred drying temperature is near that of boiling water, 212 F.Any of the condition.

standard types of dryers are satisfactory for this phosphoric acid theproduct can be kept agitated .for about 20 minutes, to prevent settingas a mass. It will then be in such physical condition as to permitgrinding in a swinghammer mill, for example, and subsequently screenedto minus 6 mesh. This 6 mesh product can then be introduced into arotary dryer and heated to a temperature of about 212 F. for 20 minutes,and discharged from the dryer in'the physical desired This product, thedouble salt (or mixed salts) of NazHaPaOmZNaI-IzPO-r, has a Slighttendency to be hygroscopic after final drying, but not nearly so much asmonosodium phosphate alone. This hygroscopic tendency can be taken careof in the usual ways, for instance, by coating with starch. Or it can bemarketed as such, notwithstanding its tendency to be slightlyhygroscopic.

Example II The same reaction as Example I is used. The 7 resultingproduct of the double (or mixed salts) can be converted by heating atthe required temperature into sodium acidpyrophosphate:

minutes the change to NazHzPzo-z is complete.

The final product. Nazl-lzPzo-z, is stable, and shows no signs of beinghydroscopic.

Example III 200 lbs. of tetrasodium pyrophosphate analyzing 53.4% P205were mixed with 194.19 lbs. of phosphoric acid analyzing 55% P205, andthe mass was kept agitated for about 20 minutes, then passed through aswinghammer mill, screened to minus 6 mesh, then dried as in Example I.or Example II, to get, respectively. sodium acid pyrophosphate-monosodum phosphate (or a mixture of the two salts) and sodium acidpyrophosphate.

It has been found that crystalline tetrasodium pyrophosphate would workjust as well, and that different strengths of phosphoric acid will alsowork just as well in carrying out the process. The reaction is dependenton the total pounds of P205 in the tetrasodium pyrophosphate being equalto the total pounds of P205 in the phosphoric acid. Either or both ofthe two products will serve as a source of the acid radical in leaveningagents, or they can be used in any manner a user might desire, their usenot being limited to leavening agents.

What is claimed is:

1. The process of producing a sodium acid phosphate product, whichcomprises mixing and reacting tetra sodium pyrophosphate with phosphoricacid using approximately one mol of the former to two mols of thelatter, and heating the product under such conditions that the materialis dried and not more than one molecule of combined water is driven offfor each two'mols of monosodium phosphate present in the said product.

2. The process of claim 1, in which the drying I temperature isapproximately 212 F.

8.The process of producing a sodium acid phosphate product whichcomprises mixing tetra sodium pyrophosphate with phosphoric acid,

15 phoric acid, using approximately one mol of the former to two mols ofthe latter to produce a I mixture of sodium acid pyrophosphate andsodium acid phosphate, and heating the resulting mixture at atemperature of approximately 400 20 to approximately 420 F. forapproximately 60 minutes.

FRANK H. PERRIN. OTIS W. ALLEN.

